Sand makes for a terrible foundation
Make sure the story makes sense and the script makes sense before you start. Cause and effect should make sense. Motivations should be pretty clear. What happens at different times and places should be obvious. The script is the blueprint, and everybody uses it. Where it’s weak, everything else is weak.
This is not to say there won’t be additional notes, of course, but they supplement the script.
(If you have a good strong personality and a small crew and can keep it all in your head, then you can afford to be a little… lax… on the planning.)
